Why is this dirt coming out of my filters?


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Neale explains to one of our readers why their filter pumps out dirt into to the fish tank after it's just been cleaned...

Q) I have a Roma 240 l tank and two Fluval 306 filters. One came with the tank and comes up through the bottom and the other stands on the floor next to the tank and comes in through the lid where I attached a spray bar. The problem occurs when turning both filters back on after cleaning. They both pump out lots of fish poo into the tank. I take out all the filter media, cleaning them in tank water. I also run tapwater through the top piece which houses the impeller, so I don’t understand why this is happening. What am I doing wrong?


A) Neale responds: One of the great secrets of using canister filters properly is to understand where the dirt goes. While most goes into the sponges and other media, it also collects in the pipework. In fact, over time, bacteria and other microbes develop their own little colonies inside those pipes, trapping yet more silt, and after a few months the pipes can become quite clogged, but it’s often not enough to impede the water flow, which is why we tend to overlook this problem.

As I’m sure you know, the flow of water through a canister filter tends to drop as the sponges and other media become increasingly clogged. What started off as a torrent can end up as a trickle within a few weeks. So, you give it a good clean, and reassemble everything.


When you’ve finished and turn everything back on, there’s suddenly a brown snowstorm of muck coming out of the spray bar. What happened was that during the weeks leading up to the time you cleaned the filter, the water current weakened, allowing the microbial mats inside the pipework to get larger and larger. After cleaning the filter, the water current increases greatly, and those mats get dislodged and washed into your aquarium.


In itself, the brown gunk doesn’t do any harm. There’s nothing growing here that you wouldn’t find in the filter or even the tank itself. But it is unsightly, and will of course go straight back into the filter, clogging up your nice clean filter media.


Luckily, filter hose cleaning brushes are available. These are like giant versions of those old fashioned bottle brushes made from wire and nylon bristles. They work in the same sort of way, being inserted into the hoses and pushed back and forth until all the muck is dislodged. It’s then easy to put the hose under a running tap and send some cold water through to wash the muck out. I’d suggest doing this outdoors if you can, so the water and dirt ends up on the lawn or flowerbed where it’ll do a good job providing some extra fertiliser for your plants while also reducing the amount of water you’re wasting. While you’re at it, don’t forget to clean the filter taps as well. These are usually easily removed and rinsed under a tap.